Potentiometry
A potentiometer is used to determine the difference between the potential of two electrodes. The
potential of one electrode—the working or indicator electrode—responds to the analyte’s activity, and
the other electrode—the counter or reference electrode—has a known, fixed potential.
reference || indicator
Ecell = Eind − Eref + Ej
http://chemwiki.ucdavis.edu
Potentiometry (zero current measurement!)
Zn(s) | ZnCl2(aq,aZn2+ = 0.0167) || AgNO3(aq,aAg+ = 0.100) | Ag
Ecell = Ec − Ea
Ecell= (EoAg+/Ag – (0.05916 / 1)log(1 / aAg+)) – (EoZn2+/Zn – (0.05916 / 2)log(1 / aZn2+))
= (0.7996 V – 0.05916 log(1 / 0.100)) – (–0.7618 – (0.05916 / 2)log(1 / 0.0167)) = +1.555 V
Potentiometry
A junction potential develops at the interface between two ionic solution if there difference in the
concentration and mobility of the ions. Consider, for example, a porous membrane separating solutions of
0.1 M HCl and 0.01 M HCl. Because the concentration of HCl on the membrane’s left side is greater than that
on the right side of the membrane, H+ and Cl– diffuse in the direction of the arrows. The mobility of H+,
however, is greater than that for Cl–, as shown by the difference in the lengths of their respective arrows.
Ecell = Ec − Ea + Ej
Potentiometry – Reference Electrodes
Hg2Cl2(s) + 2e− ⇋ 2Hg(l) + 2Cl−(aq)
Hg(l) | Hg2Cl2(s), KCl(aq, sat'd) ||
AgCl(s) + e− ⇋ Ag(s) + Cl−(aq)
Ag(s) | AgCl(s), KCl(aq,aCl− = x) ||
Potentiometry
Relationship between the potential of an Fe3+/Fe2+ half-cell relative to the reference electrodes in the
example. The potential relative to a standard hydrogen electrode is shown in blue, the potential relative to a
saturated silver/silver chloride electrode is shown in red, and the potential relative to a saturated calomel
electrode is shown in green.
Potentiometry
The existence of this membrane potential led to the development of a whole new class of indicator
electrodes called ion-selective electrodes (ISEs). In addition to the glass pH electrode, ion-selective
electrodes are available for a wide range of ions. It also is possible to construct a membrane electrode for a
neutral analyte by using a chemical reaction to generate an ion that can be monitored with an ion-selective
electrode. The development of new membrane electrodes continues to be an active area of research.
reference(sample) || [A]samp(aq,aA = x) | [A]int(aq,aA = y) || reference(internal)
Ecell = Eref(int) − Eref(samp) + Emem + Ej
where Emem is the potential across the membrane. Because the
junction potential and the potential of the two reference
electrodes are constant, any change in Ecell is a result of a
change in the membrane’s potential.
Emem= Easym − (RT / zF)ln((aA)int / (aA)samp)
Potentiometry
Schematic diagram showing a combination glass electrode for measuring pH. The indicator electrode consists of
a pH-sensitive glass membrane and an internal Ag/AgCl reference electrode in a solution of 0.1 M HCl. The
sample’s reference electrode is a Ag/AgCl electrode in a solution of KCl (which may be saturated with KCl or
contain a fixed concentration of KCl). A porous wick serves as a salt bridge between the sample and its reference
electrode.
H+ + −SiO−Na+ ⇋ −SiO−H+ + Na+
The first commercial glass electrodes were manufactured using
Corning 015, a glass with a composition that is approximately
22% Na2O, 6% CaO and 72% SiO2. When immersed in an
aqueous solution for several hours, the outer approximately 10
nm of the membrane’s surface becomes hydrated, resulting in
the formation of negatively charged sites, —SiO–. Sodium ions,
Na+, serve as counter ions. Because H+ binds more strongly to
—SiO– than does Na+, they displace the sodium ions
Ecell= K + 0.05916logaH+
Potentiometry- Solid State Membrane
A solid-state ion-selective electrode uses a membrane consisting of either a polycrystalline inorganic salt or a single
crystal of an inorganic salt. For example, one can fashion a polycrystalline solid-state ion-selective electrode by sealing a
1–2 mm thick pellet of Ag2S—or a mixture of Ag2S and a second silver salt or another metal sulfide—into the end of a
nonconducting plastic cylinder, filling the cylinder with an internal solution containing the analyte, and placing a reference
electrode into the internal solution.
Ag2S(s) ⇋ 2Ag+(aq) + S2−(aq)
Ecell = K + 0.05916logaAg+
Ecell = K − (0.05916 / 2)logaS2−
(Cd+2, Cu+2, Pb+2, Br-, Cl-, I-, SCN-, S2-
Potentiometry -Complexing Agents
Another class of ion-selective electrodes uses a hydrophobic membrane containing a liquid organic complexing agent
that reacts selectively with the analyte. Three types of organic complexing agents have been used: cation exchangers,
anion exchangers, and neutral ionophores. A membrane potential exists if the analyte’s activity is different on the two
sides of the membrane. Current is carried through the membrane by the analyte.
An ionophore is a ligand whose exterior is
hydrophobic and whose interior is
hydrophilic. The crown ether shown here
is one example of an neutral ionophore.
Ca2+(aq) + 2(C10H21O)2PO2−(mem) ⇋ Ca[(C10H21O)2PO2]2(mem)
Ecell = K + (0.05916 / 2)logaCa2+
(Ca+2, K+, Li+, NH4+, ClO4-, NO3-)
Potentiometry – Gas Sensing Electrodes
The basic design of a gas-sensing electrode is shown. It consisting of a thin membrane that separates the sample from
an inner solution containing an ion-selective electrode. The membrane is permeable to the gaseous analyte, but
impermeable to nonvolatile components in the sample’s matrix. The gaseous analyte passes through the membrane
where it reacts with the inner solution, producing a species whose concentration is monitored by the ion-selective
electrode.
CO2(aq) + 2H2O(l) ⇋ HCO3−(aq) + H3O+(aq)
aH3O+ = Ka× (aCO2 / aHCO3−)
Ecell = K′ + (0.05916)logaCO2
(CO2, HF, H2S, NH3, NO2)
Potentiometry - Quantitation
Emeas
Sensitivity is 0.059 /zi V/dec
Selectivity has to be checked.
What limits the LOD?
2.303RT
K
log ai
zi F